a) Field of the Invention
The invention is directed to an arrangement for monitoring the energy radiated by an EUV radiation source with respect to the energy variations taking place in an illumination beam path, particularly for controlling dose stability in EUV lithography for chip fabrication in semiconductor technology.
b) Description of the Prior Art
In addition to special lamps, narrow-band excimer lasers with wavelengths of 248 nm and 193 nm are currently used as radiation sources for producing microchips. Scanners based on F2 lasers (157 nm) are in development at this time.
In all photolithography processes, a mask (containing the structure to be imaged) is imaged on a wafer (semiconductor disk) in the scanner in a reduced manner (the reduction is typically 1:5). EUV radiation sources (around 13.5 nm) appear to be the most promising of the various solutions for the next generation of semiconductor lithography. Aside from the characteristics of the optical system (numerical aperture, depth of focus, aberrations or imaging errors of the lenses or mirrors), the image quality of the photolithographic process is essentially determined by how accurately the radiated radiation dose (dose accuracy) can be maintained. This dose stability is determined by:                a) pulse quantization        b) pulse-to-pulse stability        c) spatial stability of the emitting volume.        
Pulse quantization is scanner-specific. The quantity of light pulses that can fall into a moving slit during the scan varies. However, this quantity can usually be ignored.
The quantities b and c are specific to the EUV radiation source itself. An arrangement which prevents the spatial fluctuations of the emitting region or suitably takes them into account would be useful for regulating pulse energy.
The throughput of a photolithography scanner (throughput=quantity of wafers exposed per time unit) is essentially determined by the pulse energy and the pulse repetition frequency of the radiation source as well as by optical losses in the scanner itself. Optical losses occur due to the limited reflectivity of the collector and mirrors and due to so-called geometric losses. The amount of radiation output that can be caught by the collector optics is defined by a quantity specific to the radiation source, the so-called source etendue (the magnitude of the emitting region [mm2] times the usable solid angle [sr]). Strictly speaking, the etendue is determined by the total geometry of the radiation source, by the dimension of the source location, by any exit windows and by the aperture of the optics following it. The etendue quantity also determines how much radiation can be detected by an optical system arranged after it. Further, the etendue represents a measurement of the radiation losses given by the geometric ratios of the radiation source (geometric losses).
To prevent geometric losses, the source etendue may not be greater than the etendue of the imaging system in the scanner, as is described in M. Antoni et al., “Illumination optic design for EUV Lithography”, Proc. of SPIE, Vol. 4146, August 2000). If this condition can be met, the radiation output in the wafer plane is now only dependent on the reflectivity of all mirrors. Regulation of the pulse energy would require detection of this pulse energy through constant measurement (monitoring) by a radiation detector.
However, the scanner optics require an isotropic radiation characteristic with respect to angular distribution. Therefore, mirror optics which couple out light are not desired in the illumination beam path for coupling out at least a part of the radiation to an energy detector. For this reason, previously known EUV radiation sources are usually operated without regulation because measurements impair the energy flux during operation.